High energy density capacitors are known for use in mobile devices and various power systems. Polymer film capacitors utilizing biaxially oriented polypropylene (BOPP) have been used for the past few decades because of their exceptionally high electric field strength of between 600-700 V/μm. See J. Ho, R. Ramprasad, and S. Boggs, IEEE Trans. Dielectr. Eletr. Insul. (2007), 14 1295-1301. Such a polymer capacitor is limited by its low dielectric constant k=2.2 (1 kHz) and operating temperature (−55 to 65° C.). Ceramic dielectrics have been used as an alternative to polymer dielectrics as they have a high dielectric constant (k), but they have lower electric field strength. Research into high energy density materials has revealed that materials having a high dielectric constant and high electric field strength, which are main aspects of increasing potential energy density, have increasing limitations at high fields. Energy density is directly proportional to the square of the electric field breakdown. Among the factors that lead to catastrophic breakdown in dielectric-based capacitors are the tunneling currents. The accumulation of charge carriers at the metal/dielectric interface lowers the energy barriers, which causes the charges to tunnel through the dielectric, leading to breakdown. This, in turn, reduces the charging and discharging efficiency of the capacitor. In addition, non-uniformities and/or defects in the metal electrodes applied directly to the capacitor dielectric can potentially produce high energy fields around the non-uniformity or defect, reducing the electric field breakdown.
Studies have shown that at very high electric fields, the leakage current sharply increases with field due to tunneling, resulting in reduced efficiency of the device. See Chen et al., “High Field tunneling as a limiting factor of maximum energy density in dielectric energy storage capacitors,” Applied Physics Letters 92, 142909, 2008. Increasing the maximum field may not increase the energy density of the device. High fields may also induce large hysteresis loops as investigated by Zhu et al., “Novel Ferroelectric Polymers for High Energy Density and Low Loss Dielectrics,” Macromolecules (2012), 45, 2937-2954, which showed that the efficiency of devices drops off at high fields. Thus, while high energy density capacitor materials can be influenced by increasing dielectric constant and dielectric strength, performance limitations exist at high fields.
It would be desirable to increase the electric field strength of a polymer dielectric to achieve low volume and low weight to meet the current and future needs for high energy density embedded capacitors. Accordingly, there is a need for an improved high energy density capacitor for use in a number of applications having increased dielectric strength which is not subject to tunneling.